The present invention generally relates to magnetic reproducing systems for reproducing a two-valved code (sign) signal recorded on a magnetic recording medium such as a magnetic tape, and more particularly to a magnetic reproducing system for a digital signal capable of finely reproducing the recorded two-valved code signal by accurately detecting the signal level.
Conventionally, a partial response system is known as one of the digital transmission systems. In this partial response system, level detection is performed with respect to a code signal received in a fine state as a code signal of a format different from that of a transmitted code signal, by taking into account the transmission characteristic of the transmission path. After performing the level detection, the code format of the reproduced code signal is restored into the code format of the transmitted original code signal. The above partial response system positively utilizes the interference between codes. Thus, the partial response system has conventionally been employed in digital transmission using a transmission path of relatively poor transmission characteristic.
On the other hand, in a magnetic recording and reproducing apparatuses, the response in the high-frequency range is quite poor upon recording and reproduction. In addition, the winding of a magnetic head shows a differentiating characteristic upon reproduction. As a result, the low-frequency component which is close to a DC component is greatly attenuated, and further, the high-frequency component is also attenuated. Accordingly, a method was conventionally used in which the partial response system is applied to the magnetic recording and reproduction of the digital signal, to record a digital signal of a code format in conformance with the magnetic recording and reproducing characteristic, and the digital signal is reproduced by discriminating the level of the reproduced signal waveform regardless of the amplitude deviation in the reproduced signal.
If the above digital signal is a signal obtained by subjecting an analog audio signal to digital pulse modulation and this digital signal is to be recorded onto a magnetic tape by a fixed magnetic head, it becomes necessary to perform the recording with a fairly high tape speed in order to obtain a desired transmission rate, because the gap width of the magnetic head cannot be set to a width narrower than a predetermined value due to limitations from the manufacturing point of view. However, if the tape speed is set to the above fairly high speed, the recording and reproducing time is greatly shortened since the length of the magnetic tape is limited. For example, it will be impossible to obtain a recording and reproducing time in the ordeer of one hour which is necessary for recording and reproducing a stereo program. Hence, the tape speed is reduced to 7.1 cm/sec, for example, to obtain the desired recording and reproducing time. Moreover, the above digital signal is divided and recorded onto a plurality of tracks (multi-tracks) along the longitudinal direction of the magnetic tape so as to reduce the transmission rate to a rate which can be transmitted when the tape speed is reduced.
In a conventional system for reproducing a magnetic tape recorded with the digital signal on multi-tracks, the high-frequency component attenuated during the magnetic recording and reproducing process of the signal reproduced from each track by the magnetic head, is compensated. In addition, a three-valued code signal is obtained according to the partial response system. The signal levels "+1" and "-1" of this three-valued code signal are converted into the level "+1", while the signal level "0" is maintained as the level "0", to convert the three-valued code signal into a two-valued code signal. Then, a control voltage which follows with an appropriate time constant even when there is amplitude deviation in the reproduced signal, is generated. Thereafter, the levels of the two-valued code signal and the above control voltage are compared in the conventional reproducing system, to restore and obtain the original two-valuedcode signal recorded upon recording.
However, in the above described conventional digital signal magnetic reproducing system, there is no self-clocking signal within the signal reproduced according to the partial response system. Thus, there was a disadvantage in that the circuit became complex, because of the need to perform a special signal processing to obtain a clock signal which is necessary when converting the three-valued code signal into the two-valued code signal. In addition, jitter is inevitably introduced within the reproduced signal due to irregularities in the tape travel and the like upon recording and reproduction, and it became difficult to match the timing of the above clock signal and the three-valued code signal. Further, the above three-valued code signal is supplied to one input terminal of a level comparator for obtaining the original two-valued code signal, and a reference signal is supplied to the other input terminal of the level comparator. However, because the reference signal is obtained through a smoothing circuit having a time constant determined by a capacitor and a resistor and wherein the three-valued code signal is subjected to full-wave rectification, there was a disadvantage in that the reference signal cannot follow the reproduced signal which varies instant by instant.
Moreover, if the magnetic recording and reproducing density is increased by using a magnetic head for multitracks, leak of magnetic flux is introduced between head gaps for recording and reproducing adjacent tracks of the multi-track magnetic head. Thus, crosstalk is introduced in signals reproduced from the adjacent tracks, and the signal-to-noise (S/N) ratio accordingly becomes poor, but this is the same problem introduced when the partial response system is applied. This problem is one of the elements which caused code error with respect to the accurate three-valued code signal of the reproduced signal and the level discrimination.